CODEWITHSUNDEEP
functional-programmingerlang
user9644796
user9644796

Reputation:

Does this code have a mistake?

I am just reading about syntax of Erlang, and read this implementation of while loop:

-module(helloworld). 
-export([while/1,while/2, start/0]). 

while(L) -> while(L,0). 
while([], Acc) -> Acc;

while([_|T], Acc) ->
   io:fwrite("~w~n",[Acc]), 
   while(T,Acc+1). 

   start() -> 
   X = [1,2,3,4], 
   while(X).

Is the semicolon a mistake? (4th line: while([], Acc) -> Acc;)

Upvotes: 0

Views: 104

Answers (2)

7stud
7stud

Reputation: 48599

I would write the two functions like this:

while(L) -> while(L,0). 

while([], Acc) -> Acc;
while([_|T], Acc) ->
   io:fwrite("~w~n",[Acc]), 
   while(T,Acc+1). 

   start() -> 
   X = [1,2,3,4], 
   while(X).

Using whitespace to separate the function definitions makes it clear that two different functions are being defined: while/1 and while/2.

I had no idea about [_|T]. It just comes after this part in the tutorial, so that's very confusing.

That's nearly equivalent to [H|T], which deconstructs a list into the Head and the Tail, where the Head is the first element of a list and the Tail is the rest of the list. The variable name _ means that you don't care about the variable, so you will not use it in the function body. In this case, it means that you don't care about the Head of the list, all you want is the Tail of the list. If a named variable is used in the head of a function clause, and you don't use the variable in the body of the function, then the compiler will give you a warning.

Here's an example of how deconstructing a list with pattern matching works:

-module(my).
-compile(export_all).

f([Head|Tail]) -> 
    io:format("The head of the list is: ~w~n", [Head]),
    io:format("The tail of the list is: ~w~n", [Tail]).

In the shell:

8> c(my).
my.erl:2: Warning: export_all flag enabled - all functions will be exported
{ok,my}

9> my:f([1, 2, 3]).
The head of the list is: 1
The tail of the list is: [2,3]
ok

10>

Upvotes: 1

Emil Vikström
Emil Vikström

Reputation: 91942

No. There are two functions defined here: while/1 (one argument) and while/2 (two arguments). The second one have two function bodies; which one to use is decided through pattern matching.

Upvotes: 0

Related Questions